why, why, why???

why do dogs roll on grotesque and disgustingly stinking things? Walking Billy at noon, he was drawn to a pile of leaves for a sniff and before I could react, he had flung himself full force onto the pile, rolling and squirming in a lustful paroxysm of joy. I heard the squirt of fluid from the decomposing horror beneath those leaves and suddenly he was covered with a thick wash of yellowish fluid with a stink like hell's outhouse. But it was the look in his eyes that was sickening beyond my ability to describe: he looked drunk, mouth open and panting, standing there covered in yellow goo. My baby looked lustful and wanton, as if he'd engaged in unbridled and licentious sexual behavior and could barely stand post-romp. Eeeeuuuuwwwwww does not begin to describe it. Why do they do this??